There has been an increasing interest of late in the use of flexible, collapsible containers for handling granular, liquid or powder (flowable) materials such as chemicals, minerals, fertilizers, foodstuffs, grains and agricultural products. The advantages of such receptacles include relatively low weight, reduced cost, versatility and, in the case of reusable receptacles, low return freight costs.
Fabrics are often utilized in the construction of flexible, collapsible containers where strength, flexibility and durability are important. Historically, such containers have been fabricated from natural fibers; however, in recent years synthetic fibers manufactured from polypropylene or other plastics have come into extensive use. The popularity of synthetic fibers can be attributed to the fact that they are generally stronger and more durable than their natural fiber counterpart.
Even with the advances in fabric construction from natural to synthetic fibers, fabrics in general possess qualities that render their use in certain applications undesirable. For example, the friction that occurs as flowable materials are handled by fabric receptacles tends to cause a significant build-up and retention of static electric charge within the receptacle. Discharge of the generated static electric build-up is often difficult, if not impossible, because fabrics are generally not electrically conductive materials. However, discharge is imperative as static charge potential poses a significant danger of fire or explosion resulting from a static generated electrical spark.
In an effort to address the undesirable static electric charge characteristic of fabrics, manufacturers of plastic fabrics covered one side of the fabric with a metallic foil-like layer to form a laminate. An adhesive is applied between the laminated layers to affix the foil-like layer to the plastic fabric. The foil-like layer is generally comprised of aluminum or some other electrically conductive metal. The laminated fabric is then used to construct the fabric receptacle, for example, with the foil side of the fabric comprising the interior surface. The foil layer provides an electrically conductive surface exposed to the flowable materials through which static electricity generated during material handling is discharged to an appropriate ground.
While adequately discharging static electric build-up, the foil layer in the laminate is susceptible to abrasion, tearing and separation from the fabric layer through normal use of the receptacle. For example, in filling, transporting and/or emptying of foil laminated fabric receptacles, abrasion between the flowable material and the foil layer tends to cause the foil layer to tear and/or separate from the fabric layer. The cumulative effect of such abrasion quickly reduces the effectiveness of the foil layer as a static electric discharge surface. Furthermore, tearing of the foil often results in a release of foil particles and flakes from the fabric contaminating the contained flowable materials.
To address the problems experienced with foil laminated fabrics, U.S. Pat. No. 4,833,008, issued to Norwin C. Derby discloses a metalized fabric comprised of a plastic woven base fabric laminated to a metalized plastic film. The plastic base fabric is preferably a woven polypropylene fabric, and the plastic film is preferably an extruded polypropylene film. The plastic film is metalized through a vapor deposition process whereby a thin film of electrically conductive material is deposited on one side of the plastic film. The woven plastic fabric and the metalized plastic film are then laminated together through use of a plastic adhesive. Unlike foil laminated fabrics, the thin conductive layer deposited on the plastic film is not subject to tearing or flaking.